Teams
In Team Fortress 2 Classic, two teams compete against each other: RED (Reliable Excavation Demolition) and BLU (Builder's League United). These teams make up the two teams in regular maps. There are also two more teams, GRN (Global Radio Network) and YLW (Yard Logistics Workers), which a player can join if they are playing in a Four-Team map.
In-game
During a game, players work with their team to fulfill the objectives of the game mode they're playing. In certain gamemodes such as Attack / Defense, the team's roles may be asymmetrical, in which case, RED is the defending team and BLU is the attacking team.
Some classes have weapons that are designed to interact with a player's team, for example, the Medic's Medi Gun is used to heal teammates and the Civilian's Umbrella, once ready, grants a teammate mini-crits.
Team Selection
RED
BLU
GRN
YLW
Classes
A team can consist of a maximum of 32 players on Two-Team, and 16 players on Four-Team, each being one of nine main Classes. Each class is designed to be functionally different than the others and has weapons, to fulfill a particular role within a team. All of the main nine classes are split into 3 sub-groups: Offense, Defense and Support.
Spectator
In Spectator Mode, a player can watch the match through participating players' eyes and fly around freely without actually playing the game. While a player is dead, they also spectate their own team's players.